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HF Theremin


This is a simple Theremin. It is a musical instrument which uses a pair of high-frequency oscillators, one of which can be tuned by hand capacitance to form a beat note against the other. The oscillators are closely matched in frequency, such that the resultant beat note is audible. This example has the (I believe) unique feature of being crystal-controlled, and thus is stable as soon as it is switched on. It is compact, and uses a small 9V battery. It has only enough output to power a pair of 32-ohm headphones, but that is ideal for practice and an external amplifier may be connected for performance use.

It is played by approaching the whip antenna with one hand to produce the tone, which begins at a few hertz and increases to several kilohertz when the antenna is nearly touched. Level control is effected by shading a light-dependent resistor (LDR) from the white LED mounted above it. The LED housing is the round, black blob in the photo above, and its light-spot reveals the location of the LDR. Level control is smooth and progressive, but without adding the complexity of a window-amplifier (squelch circuit), the sound never quite disappears. A 'conventional'  hand-capacity level control could be used, but the added complexity would spoil the project, in my opinion.

In common with all simple theremins, the distance / pitch relationship is exponential; but as the project is to be kept as simple as possible, any correction of this is left as an exercise for the experimenter. One thought here is to set the crystal oscillators such that they are near the limit of their 'pulling' range; here, any extra capacitance applied has an approximately logarithmic influence on the frequency change, an effect which could linearise the distance / frequency relationship.

The prototype uses a pair of 24MHz crystals. This is an extraordinarily high frequency for a theremin, but is necessary if a wide frequency variation is to be had from the 'control' oscillator. The system uses a Philips SA602 mixer-oscillator, a simple Colpitts crystal oscillator and an LM386 audio amplifier. An ORP12 LDR is used for level control. The project fits easily into a 2-ounce tobacco tin, complete with battery, all circuitry and the single on/off/volume potentiometer and switch. Other cabinet options are a wooden cigar box, and the wooden boxes which craft knife sets or other small tools are packaged in.
The circuit diagram above shows only the tone-generation circuitry; you will need to add an audio-frequency amplifier for a complete system. The author recommends the LM386 for personal use, but a quieter amplifier would be advisable for performances. The NE5534 is a suitable low-noise wideband operational amplifier for making a high-quality amplifier.

Please let me know at http://g1inf.blogspot.com if you have any suggestions or modifications to this design.
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